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What is the biggest true apo?

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#26 noisejammer

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Posted 11 July 2020 - 04:29 PM

Peter Wise' Zerochromat design is essentially colour free. Several years back, I did some FEA for him that showed a 50" model might be possible.... if you can find a customer and a stress free lump of BK7 to function as the primary.

 

On the question of mass, the tube of the Johannesburg 26" f/14 instrument weighs 4.5 tonnes and it's a lowly achromat. :)



#27 TNmike

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Posted 11 July 2020 - 04:37 PM

What are the largest APOs using premium glass (flourite like) such as FPL53 or FDC100?


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#28 Rollo

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Posted 11 July 2020 - 04:43 PM

What are the largest APOs using premium glass (flourite like) such as FPL53 or FDC100?

I heard that Takahashi will make custom size large refractors ,,,, but the cost might be out of sight !   lol.gif


Edited by Rollo, 11 July 2020 - 04:44 PM.


#29 Wildetelescope

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Posted 11 July 2020 - 04:44 PM


I saw that at NEAF as well. An inch impressive beast.

Jmd
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#30 payner

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Posted 11 July 2020 - 07:14 PM

I heard that Takahashi will make custom size large refractors ,,,, but the cost might be out of sight !   lol.gif

Maybe not the biggest, but ... http://astrosurf.com...em3500_2000.jpghttp://astrosurf.com...shi/FCT-250.gif


Edited by payner, 11 July 2020 - 07:15 PM.

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#31 payner

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Posted 11 July 2020 - 07:28 PM

Then there's this https://www.takahash.../en/FET-300.php. One can dream.


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#32 Spikey131

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Posted 11 July 2020 - 08:02 PM

The "Rich" and "just because"........

Really?  Is that it?  Is there a scientific need for such a scope or is it just an expensive toy?



#33 Supernova74

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Posted 12 July 2020 - 05:27 AM

well if your looking for a classic observatory class Achromatic refractor doublet around F7 which I consider due to its size of 224mm clear aperture an Italian company called technosky manufactures them for a bargain price of £5400 in aperture to price ratio I think that’s considered a bargain regarding a refractor I think an APM APO would most probably cost 6-7 times the amount ok superior optics however thay do exsist.



#34 junomike

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Posted 13 July 2020 - 04:02 PM

Really?  Is that it?  Is there a scientific need for such a scope or is it just an expensive toy?

IMO there's better designs which can be utilized for scientific needs. 

I'm sure there's a few OB that may use a large Refractor but if you look at all the HUGE telescopes in use and you'll see that all are of other design ~ mostly mirrors due to cost.



#35 Richard Whalen

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Posted 13 July 2020 - 09:48 PM

I think it would be pretty tough to make a true APO larger than 9" or 10" unless you have a fr  f12 plus or go with 4 lens design. Maybe a triplet would get you there at 8" around f9 or longer or if you used fluorite. At least what my definition of a true APO is. I bet that 20" would have color well outside the airy disc in a spot diagram?



#36 Eliserpens

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Posted 17 July 2020 - 10:38 AM

IMO there's better designs which can be utilized for scientific needs. 

I'm sure there's a few OB that may use a large Refractor but if you look at all the HUGE telescopes in use and you'll see that all are of other design ~ mostly mirrors due to cost.

I thought there was a glass stability issue too.  A mirror can be made as thick as you like, a lens has limits - and glass will flow over time....


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#37 Astrojensen

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Posted 18 July 2020 - 02:30 AM

and glass will flow over time....

This has been shown to be a myth. Glass does NOT flow over time. It is stable over millions of years. 

 

The reason that many very old windowpanes are thicker at the bottom than the top, is due to the manufacturing process, where a large clump of glass was spun into a thin, lens-shaped disk as it cooled, and then cut into suitable pieces. It follows automatically that such a disk is thicker near the center, and the pieces you get are not uniformly thick. The craftsmen that built the windows simply installed the pieces with the thickest part downwards, as anyone would do, because that's the easiest to balance.

 

 

Clear skies!

Thomas, Denmark 


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#38 Astrojensen

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Posted 18 July 2020 - 02:44 AM

  A mirror can be made as thick as you like, a lens has limits

It's true that a large lens has flexure, but the rear surface of the lens almost perfectly cancels the aberrations from the front surface! 

 

In ideal seeing conditions, the Yerkes and Lick refractors have both been regularly used at 1000x - 3000x, when examining extremely narrow double stars and both can split doubles down to their Dawes limit, which eminently show that the limit of lensmaking doesn't stop at 40", as is so often claimed.  

 

But in physical size, a refractor larger than 40" is completely impractical, unless, of course, someone invents a design that can be made to much lower f/ number. Currently, that I know of, it's not possible to make special glass types normally used in apochromats in sizes much larger than ~12". The dialyte design shows promise, but still needs to be rather long, to keep other aberrations at bay. Peter Wise calculated a 60" design, that on paper should work well, but actually building it is of course another matter. 

 

 

Clear skies!
Thomas, Denmark


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#39 Jon Isaacs

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Posted 18 July 2020 - 06:30 AM

It's true that a large lens has flexure, but the rear surface of the lens almost perfectly cancels the aberrations from the front surface!

 

 

I'm wondering about that.. my sense as an engineer is that the sag would only be symmetrical with a lens that was horizontal, i.e., viewing at the zenith.  

 

Do you have a reference?

 

Of course, these days, there's little reason to build large refractors with all the issues the present.

 

Jon


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#40 BillP

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Posted 18 July 2020 - 07:51 AM

The "Rich" and "just because"........

The scientists at the U.S.N.O. use the vintage 26" Clark refractor every single clear evening for scientific study.  Actually though, a "tool" never does science, only the person wielding the toll can actually do science grin.gif  So any tool that works for the intended purpose, is just fine.


Edited by BillP, 18 July 2020 - 07:52 AM.


#41 Astrojensen

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Posted 18 July 2020 - 08:40 AM

I'm wondering about that.. my sense as an engineer is that the sag would only be symmetrical with a lens that was horizontal, i.e., viewing at the zenith.  

 

Do you have a reference?

 

No, but I've read it many times in many different places over the years. 

 

Thinking about it, we can infer by some logical thinking, that it must be true, as the lenses of the great Yerkes and Lick refractors are both less than 2" thick and a mirror of that size and thickness will have aberrations of many dozens of wavelengths, if only supported along the edge, and not give diffraction limited performance, while both large refractors have resolved double stars to 0.2" and some below. 

 

 

Clear skies!

Thomas, Denmark



#42 j.gardavsky

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Posted 18 July 2020 - 09:36 AM

Glass has its viscous flow,

https://link.springe...023995.15871.ca

 

We call it rheology,

JG


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#43 Eliserpens

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Posted 18 July 2020 - 09:44 AM

This has been shown to be a myth. Glass does NOT flow over time. It is stable over millions of years. 

 

The reason that many very old windowpanes are thicker at the bottom than the top, is due to the manufacturing process, where a large clump of glass was spun into a thin, lens-shaped disk as it cooled, and then cut into suitable pieces. It follows automatically that such a disk is thicker near the center, and the pieces you get are not uniformly thick. The craftsmen that built the windows simply installed the pieces with the thickest part downwards, as anyone would do, because that's the easiest to balance.

 

 

Clear skies!

Thomas, Denmark 

Thanks for straightening (excuse the pun, but it is apropos... ) that out for me Thomas.  Another high-school 'fact' corrected.  But I bet I am not alone!  I found this:

 

"Speaking of time, just how long should it take—theoretically—for windows to thicken to any observable extent? Many years ago, Dr. Chuck Kurkjian told me that an acquaintance of his had estimated how fast—actually, how slowly—glasses would flow. The calculation showed that if a plate of glass a meter tall and a centimeter thick was placed in an upright position at room temperature, the time required for the glass to flow down so as to thicken 10 angstrom units at the bottom (a change the size of only a few atoms) would theoretically be about the same as the age of the universe: close to ten billion years. Similar calculations, made more recently, lead to similar conclusions. But such computations are perhaps only fanciful. It is questionable that the equations used to calculate rates of flow are really applicable to the situation at hand."

 

at Corning Glass...
https://www.cmog.org...does-glass-flow


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#44 Freakshow

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Posted 29 July 2020 - 11:25 PM

So who do you think buys a 20” f12 6500mm refractor?  And for what application?

My hunch is the Vatican.   They have a serious astro program.



#45 dothead

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Posted 30 July 2020 - 05:36 AM

No, it's an Italian millionaire living in the vicinity of Rome.

 

BTW, the largest 100% color corrected refractor is the 1 meter Swedish Solar Telescope of Schupmann design.



#46 Freakshow

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Posted 30 July 2020 - 11:40 AM

What does the field flattener look like for a one meter scope?  Thoughts on a one-meter Petzval versus a triplet.



#47 RogerRZ

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Posted 30 July 2020 - 12:00 PM

What does the field flattener look like for a one meter scope?  Thoughts on a one-meter Petzval versus a triplet.

Or is the mating element FPL53? If it isn't, I'm putting my check book away.



#48 Nippon

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Posted 30 July 2020 - 10:11 PM

Pretty easy to see why mirrors rule above 6 to 8 inches


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#49 N-1

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Posted 30 July 2020 - 10:26 PM

I suspect that if you can afford a custom made LZOS 510mm aperture triplet, you can also afford to condition the air inside the observatory to a couple degrees below ambient, mitigating thermal issues with the objective.  I'd like to see some pictures of its installation though.

 

I do know Markus also did a 12" LZOS bino-scope.

 

Jeff

I'd expect the OTA to have its own internal HVAC system, keeping it at ambient temperature in real-time grin.gif



#50 Arcamigo

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Posted 30 July 2020 - 11:10 PM

It seems to me like it might be cheaper to make a mount to hold 15 AT130EDTs and then find a way to combine their light interferometrically.




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